John Sayer's Rear Wall Absorbers

Hi, Like many have already: thanks for this wonderful resource! I'm going to build the Rear Wall Absorber referenced on this page: http://www.johnlsayers.com/HR/index1.htm I have one question: the rear 1/4" plywood panel is labled 7' x 4'. The diagram also specifies to build to the height of the room. Does this mean that the absorber is 7' wide but only covered in the back 4' down from the ceiling and the rest is left open? Or is the plywood backing supposed to close the box entirely? If so the 1/4" plywood will have to be more than one sheet... Thanks for any clarifications! Steve

OK... so perhaps according to the "DON'T EVEN think ABOUT POSTING before you READ THIS..." thread, I haven't been forthcoming enough with information, although frankly I'm not sure it's very relevant, but here goes: I have a 13' W x 18.4' L x 8.1' H that I intend in the long term to strip and remodel into a proper acoustic workspace. For now my intention is simply to deaden the reverberation of the gysum walls, ceiling and linoleum-over-wood flooring. and hopefully, though probably lamely, tame some of the modal resonances. I've read enough on this board to know that what I'm doing is very incomplete, but the room is so live I'm sure anything will help. My intention is not to have a perfect acoustic space but at least to have comfortable one for composition and arranging, which are my primary goals. I was able to get some Primacoustic foam paneling from a studio closeout for free, and I'm aware of the limitations of this material (I've read the threads about how real studios don't use this stuff...); I'm using it as a temporary solution. I have 3 of the big triangular pieces they refer to as "Australis Bass Traps" (!) which I plan to place on either side and above the speaker end of the space. I also have quite a few pieces I intend to use on the sides and perhaps as a cloud over the console/listening position. The desk listening position faces a double-glass patio door and the rear wall absorbers on John Sayer's Wall Unit page (link in previous post) seem like good generic absorbers to put between this door and the speakers for helping control reflections from the door and deaden some of the room's reverberation. When I do rebuild/remodel the space, I figure I can always use them as general purpose gobos. After spending some of the intervening time since my last post studying the diagram on that page, my question is actually quite simple: What is the length left to right of the rear wall absorber as drawn on the webpage? More specifically, is the assembly 7 feet wide and 4 feet tall, or 7 feet tall and 4 feet wide? In the former, each open section would be 3.5' by 4 while in the latter it would be 2' x 7'. Does it matter either way? Another question about these: the back is specified as 1/4" plywood. Would masonite, being more flexible and reactive, do a better job? Just trying to nail down the specifics. Thanks! Steve

What you see on that page is pretty much to scale; note the 8" dimension for depth (not critical, deeper is better) - each half of the plan shown is a 2 foot wide bay, same as rigid insulation typically comes. The total width as shown is 4 feet, the 7 foot height is just so you can actually move it into place in a room with 8 foot ceilings. If you have a way to get it in place, you can make them however tall will fit. Another option is to make them half height, minus maybe a half inch each; then you can stack two units - they'll be lighter and easier to handle. Don't use masonite, in fact a heavier back will keep the trap from trying to emulate a resonant panel absorber, which it is NOT. I'd tend to go for a back of 3/4 MDF instead. If you have any scraps of insulation left over, putting them inside the traps behind the main pieces of fiberglass is a good idea... Steve

Thanks for the details, Steve! One more clarification: Would each bay then be 2' x 4' or 2' x 7'? In other words #1: can I put a MDF cross-piece to reinforce the box midway from the top that completely separates each bay into 2' by 4' sections, or should I put a partial brace to keep the top and bottom 2' x 4' sections connected? In other words #2: Is there any benefit to having a 2' x 4' bay vs a 2' x 7' bay in this design? Is there perhaps more low frequency attenuation keeping the airspace behind the insulation large? In other words #3: Is the point of this design to have multiple sealed 2' by 4' boxes? In that case, couldn't these units be assembled with 2' x 4' boxes, making them easier to handle for non-permanent installations and for experimentation? You do mention stacking half-size units, so... I'm about to put this thing together and since I have options, I'd like to be sure what they are. Very grateful for your time and feedback! Steve

You're over-thinking this; I know, because it's where I was about 10 years ago... For a rear trap, all that's necessary is for the absorbent to be away from the rear wall by up to 3 or 4 FEET, if you can afford the space. Whether you build several smaller boxes with mineral wool/rigid fiberglass across the fronts, or one LARGE one, makes no difference acoustically - all that matters is the thickness of the absorbent (more is better, 4" is about minimum) the density of the absorbent (3 PCF is ideal, up to (maybe) 8 PCF but that's pushing it, and I'd face that dense a material with softer stuff), and the distance from the front of the absorbent to the solid wall behind. IF you intend to move stuff later, I'd be more tempted to build backless frames (gobo's) and just stuff 'em with rockwool and cover with cloth; then you can play with depth from the wall and somewhat tune your space. Until you get into building tuned absorbers such as Helmholtz/slat/slot resonators and panel traps, hermetic sealing isn't necessary at all. If you still wanna build boxes like the SAE plan, each box can be 4x4 feet (slightly shorter if your ceiling is exactly 8 feet) so you can stack two - if you build four of these, you'll cover a space 8 feet tall by 8 feet wide. Each "bay" inside the boxes should be 2' x 4', so it will fit normal insulation batts. And finally - if your space is as isolated as you need it, you might consider just framing out the rear of the room and building a full-width, full-height rear trap - it's easier than building several boxes, and won't tend to rattle if you get too loud... Steve